1. Field of the Invention
The present invention relates to a coarse frequency synchronization method and apparatus for an orthogonal frequency division multiplexing (OFDM)—based system, and more particularly, to a coarse frequency synchronization method and apparatus in an OFDM receiver.
2. Description of the Related Art
FIG. 1 is a block diagram showing the structure of a conventional OFDM receiver system. Referring to FIG. 1, the conventional OFDM receiver system includes an OFDM demodulator 10 and a frequency synchronizer 12. The OFDM demodulator 10 includes a RF receiver 101, an analog-to-digital converter 102, an I/Q filter 103, a frequency corrector 104, an FFT unit 105, and a Viterbi decoder 106. The frequency synchronizer 12 includes a buffer register 121, a complex data multiplier 122, an IFFT unit 123, a maximum value detector 124, a counter 125, and a reference symbol generator 126.
The operation of the conventional OFDM receiver system of FIG. 1 will now be described. First, when the RF receiver 101 receives a RF signal, the analog-to-digital converter 102 quantizes the received RF signal. The I/Q filter 103 then separates In-phase and Quadrature (I/Q) component signals from the quantized signal. The frequency corrector 104 corrects a frequency error of the filtered signal, and the FFT unit 105 receives the frequency corrected signal and performs a Fast Fourier Transform in order to demodulate the signal. A Viterbi decoder 106 decodes the demodulated signal.
The demodulated signal is stored in the buffer register 121 of the frequency synchronizer 12 and output as a reception signal X. A phase reference symbol output from the reference signal generator 126 is denoted by Z. If the reception signal X has a frame synchronization error of ω and the k-th signals of X and Z are Xk and Zk, respectively, then Xk=Zke−J2πkw/N. The complex data multiplier 122 multiplies the complex conjugate of Zk by Xk, an inverse FFT (IFFT), denoted by hn, is performed on the result of the multiplication, as shown in Equation 1 below, and then the result of the IFFT is output.
                              h          n                =                  IFFT          ⁢                      {                          XZ              *                        }                                              (        1        )                                                          ⁢                  =                                    1              /              N                        ⁢                                          ∑                                  k                  =                  0                                                  N                  -                  1                                            ⁢                                                          ⁢                                                X                  k                                ⁢                                  Z                  k                  *                                ⁢                                  ⅇ                                      j                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    k                    ⁢                                                                                  ⁢                                          n                      /                      N                                                                                                                                                                                        ⁢                  =                                    1              /              N                        ⁢                                          ∑                                  k                  =                  0                                                  N                  -                  1                                            ⁢                                                          ⁢                                                Z                  k                                ⁢                                  ⅇ                                                            -                      j                                        ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    k                    ⁢                                                                                  ⁢                                          w                      /                      N                                                                      ⁢                                  Z                  k                  *                                ⁢                                  ⅇ                                      j                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    k                    ⁢                                                                                  ⁢                                          n                      /                      N                                                                                                                                                                                        ⁢                  =                                    1              /              N                        ⁢                                          ∑                                  k                  =                  0                                                  N                  -                  1                                            ⁢                                                                                                                                                      ⁢                                          Z                      k                                                                            2                                ⁢                                  ⅇ                                      j                    ⁢                                                                                  ⁢                    2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                                                                  k                        ⁡                                                  (                                                      n                            -                            w                                                    )                                                                    /                      N                                                                                                                                                                                        ⁢                  =                      δ            ⁡                          (                              n                -                w                            )                                                                      
In this case, the reception signal Xk containing Δfi, which is an integer multiple of frequency error with respect to the transmission signal, is equal to Zk−Δfe−j2πkw/N. Thus, Equation (1) can be rewritten as Equation (2):
                                                                        h                n                            =                              IFFT                ⁢                                  {                                      XZ                    *                                    }                                                                                                                                                    ⁢                              =                                                      1                    /                    N                                    ⁢                                                            ∑                                              k                        =                        0                                                                    N                        -                        1                                                              ⁢                                                                                  ⁢                                                                  X                        k                                            ⁢                                              Z                        k                        *                                            ⁢                                              ⅇ                                                  j                          ⁢                                                                                                          ⁢                          2                          ⁢                                                                                                          ⁢                          π                          ⁢                                                                                                          ⁢                          k                          ⁢                                                                                                          ⁢                                                      n                            /                            N                                                                                                                                                                                                                                                                  ⁢                              =                                                      1                    /                    N                                    ⁢                                                            ∑                                              k                        =                        0                                                                    N                        -                        1                                                              ⁢                                                                                  ⁢                                                                  Z                                                  k                          -                                                      Δ                            ⁢                                                                                                                  ⁢                                                          f                              i                                                                                                                          ⁢                                              ⅇ                                                                              -                            j                                                    ⁢                                                                                                          ⁢                          2                          ⁢                                                                                                          ⁢                          π                          ⁢                                                                                                          ⁢                          k                          ⁢                                                                                                          ⁢                                                      w                            /                            N                                                                                              ⁢                                              Z                        k                        *                                            ⁢                                              ⅇ                                                  j                          ⁢                                                                                                          ⁢                          2                          ⁢                                                                                                          ⁢                          π                          ⁢                                                                                                          ⁢                          k                          ⁢                                                                                                          ⁢                                                      n                            /                            N                                                                                                                                                                                                      (        2        )            
The result of Equation (2) is obtained using the process for calculating convolutions of two signals in the time domain, and the resulting value hn is a channel impulse response (CIR). In this way, the OFDM receiver system that recognizes a frequency domain reference signal can obtain the CIR using the sent reference symbol. Here, if the signal Zk from a sending end is given as a pseudo noise (PN) sequence so that it has no correlation with k, the result of Equation (2) is a maximum peak value if a frequency error is zero and a sequence of small values which correspond to noise if a frequency error is not zero.
Using this relationship, a maximum value detector 124 detects a maximum peak value by applying Equation (2) after cyclically rotating the reception signal Xk by Δfi, and the counter 125 finds a rotation amount Δfi where the maximum peak value occurs. The rotation amount Δfi is an error Fo that is an integer multiple of a frequency error to be detected. The relationship is defined by Equation (3):
                              F          o                =                                                            max                                                                                      Δ                  ⁢                                                                          ⁢                                      f                    i                                                                                ⁢                      {                          max              ⁢                                                          ⁢                              amp                ⁡                                  [                                      IFFT                    ⁢                                          {                                                                        X                          R                                                ⁢                        Z                                            }                                                        ]                                                      }                                              (        3        )            where Z denotes a phase reference symbol and XR denotes a reception symbol made up of terms of X((k+Δfi))N obtained by cyclically rotating an individual value Xk of a reception symbol X in the frequency domain by Δfi.
A coarse frequency synchronization method according to the conventional OFDM receiver system discussed above makes it possible to actually or theoretically estimate a frequency error in any situation regardless of channel environment and frame synchronization error. However, this method requires a considerable amount of computation. Specifically, this method requires a very complicated IFFT module in order to accurately estimate a frequency error, and causes an excessive time delay due to a long response time.